How Radio Waves Transmit Information in Wireless Communication

Mobile phones, Wi-Fi networks, FM radio and Australia’s Emergency Alert service all depend on the same basic capability: turning electrical changes into waves that can cross the air without cables. A wave carries the energy, but the information is represented by deliberate changes to that wave.

The practical challenge is to create those changes, send them efficiently, detect them at the other end and distinguish them from other transmissions. This process links physics, electronics and computing, from a classroom transmitter to mobile networks used in Australian cities and remote communities.

From currents to electromagnetic waves

A radio wave is an electromagnetic wave: linked electric and magnetic fields that vary over time and spread outward from a source. The fields change direction repeatedly, with the oscillation frequency measured in hertz, meaning cycles per second. Wavelength is the distance between successive crests, and the wave travels in air at close to 300,000 kilometres per second.

A 100 MHz FM radio signal therefore has a wavelength of about three metres. Higher frequencies have shorter wavelengths. Radio waves do not need a material medium, unlike sound, which relies on particles to carry its vibrations. The ideal gas law describes relationships between a gas’s pressure, volume and temperature, whereas electromagnetic waves consist of changing fields rather than moving particles.

Antennas send and receive radio signals

A transmitter uses an oscillator to produce alternating current at a chosen carrier frequency. Current flowing through an antenna creates changing electric and magnetic fields that radiate away from the wire. Designers choose the antenna’s length, shape and position so that it couples efficiently to the intended wavelength and directs energy towards the intended receivers.

Reception reverses the process. The arriving field induces a tiny alternating voltage in the receiving antenna, and a resonant circuit can be tuned to favour one frequency while rejecting others. This filtering matters because a city radio environment contains many stations. Without selectivity, the receiver would collect all of them at once rather than isolating the broadcast it needs.

Modulation gives the carrier meaning

A steady carrier alone does not contain a voice, image or data stream. Modulation connects information to the carrier by changing one of its properties: amplitude, frequency or phase. A microphone turns sound into an electrical signal, and an AM broadcast varies the carrier’s strength in step with that signal. FM instead encodes audio in small frequency variations, which usually gives better resistance to many forms of electrical noise.

Digital systems first turn sound, video and files into bits. Network equipment then maps those bits onto symbols that change the carrier’s amplitude, phase or frequency. More symbols per second can carry more data, but they require more bandwidth. A practical transmitter therefore combines a carrier generator, a modulator, processing circuitry and an antenna rather than relying on an antenna alone.

Receivers recover the original information

The receiver’s antenna produces an alternating electrical signal, usually very small. A tuned circuit selects the wanted station, amplifiers raise the signal, and a demodulator recovers the information-bearing changes. For broadcast audio, a speaker then reproduces the sound. For mobile data, software separates headers, payload and control information, checks for errors and passes usable data to the device.

Detection depends on measurable change, not simply on a wave being present. The chemistry behind autumn leaf colours offers a useful cross-disciplinary comparison: scientists infer seasonal chemical changes from the light reflected by leaves. A radio receiver instead infers a message from changes in electrical strength, frequency or phase. Although leaves are not transmitters, both examples show how an observed pattern can reveal a process.

Many users share a limited spectrum

Radio frequencies are finite, so regulators divide them into bands with different purposes. In Australia, the Australian Communications and Media Authority licenses and allocates spectrum for mobile services, broadcasting and other users. Wi-Fi equipment uses unlicensed bands, including 2.4 GHz and 5 GHz, within rules that limit interference and transmission power.

Within a licensed mobile network, base stations serve geographical cells. Nearby cells can reuse the same frequencies with coordinated power and scheduling to limit interference. A phone selects a suitable base station and may hand over its connection as it moves between cells. This technology supports the prepaid SIMs and 4G or 5G plans offered by carriers such as Telstra, Optus and Vodafone, including in major markets such as Melbourne and Sydney.

Australian distances and interference

Radio waves weaken as they spread, and buildings, hills and vegetation can absorb, scatter or reflect them. Lower-frequency signals generally diffract around obstacles more readily and can cover greater distances, while higher frequencies can support wider channels but are more easily blocked. Australia’s Mobile Black Spot Program funds new or upgraded mobile coverage in regional and remote communities, where large distances and sparse tower placement create difficulties.

For a very different kind of field, Earth's magnetic field surrounds the planet and helps shield it from charged particles; a mobile radio signal is a much weaker, deliberately changing field generated by a transmitter. Australia’s Square Kilometre Array project in Western Australia also depends on keeping parts of the radio spectrum quiet for radio astronomy. Protecting those bands shows that a frequency can be valuable for communication, science and emergency services at the same time.

Investigate radio waves around you

Begin with an ordinary radio and Australia’s 88–108 MHz FM broadcast band. Record several stations and their frequencies, then estimate each wavelength using wavelength = speed ÷ frequency. A school-approved spectrum analyser can also show which nearby Wi-Fi channels are active. Sketching helps: draw a regular carrier, then mark the changes in amplitude, frequency or phase that would represent a selected piece of information.

Now put your learning into action: create a one-page explanation of how a radio signal can represent a voice, and label encoding, modulation, transmission, reception and decoding on a simple signal-path diagram. Include one Australian example, such as an FM broadcast or a mobile call, and explain how interference could distort the message. Present the finished explanation and labelled diagram to your study group.